1. Field of the Invention
The present invention relates to a damping pad for preventing brake squeal, which particularly serves to improve an effect of preventing the brake squeal.
2. Description of the Related Art
In conventional brakes, particularly in disc brakes, sliding between brake shoes and disc rotors may cause vibration of the rotors and brake shoes in braking operations, which causes vibration of metal plates fixed to the brake shoes and vibration of other components of the disc brakes such as a calipers, resulting in uncomfortable noises or squeal. In order to prevent or suppress such squeal, attempts have been made, as shown in Japanese Unexamined Utility Model Publication No. Sho 61-200936, in which a viscoelastic material is fixed to a metal plate of a brake pad to damp the vibration. With this pad, the damping can however be effectively performed only in a restricted range, and sufficient damping efficiency cannot be obtained.
The inventors of the present invention have specifically studied a cause of brake squeal and have investigated an effective way to suppress or prevent the squeal. As a result, it has been found that the brake squeal cannot be prevented in the prior brake pads due to the fact that an operating temperature range of the brake has not been taken into consideration at all. That is, the temperature range of the brake varies depending on operating conditions. Immediately after the start of a vehicle in a cold condition, the above mentioned viscoelastic material may be kept at a temperature of 0.degree. C. or less nearly the same as an ambient temperature. In hard braking during a high speed driving, the temperature of the above viscoelastic material may increase over 100.degree. C.
However, in the prior brake pads, a loss factor of the viscoelastic material is kept at a high value enough to effectively damp the vibration only in a restricted temperature range as shown in FIG. 4, so that the sufficient damping efficiency cannot be kept through a wide temperature range in which the brake may operate, and a temperature region in which the vibration is insufficiently damped is inevitable, resulting in such a problem that the generation of the brake squeal cannot be prevented. The term loss factor is known in the field of viscoelastics wherein the loss factor of a material is considered as a function of temperature and frequency. In the field of viscoelastics, by using the loss factor of a material .eta., complex elasticity is defined as follows: EQU E=(1+j.eta.)
wherein j.sup.2 =-1 (j is a imaginary number unit) and E is an elasticity constant (Young's modulus or shearing elasticity factor). .eta. is physically a dimensionless amount showing the degree of damping efficiency.
Loss factor in a vibration system can be calculated from a frequency response function (FRF) obtained from known vibration tests, which are known in the field of viscoelastics.